31 research outputs found
Towards a physical model for galactic rotation curves
Extensive and meticulous observations of the rotation curves of galaxies show that they are either flat or gently going up, but rarely decreasing, at large galactocentric distances. Here we show that the gravitational potential which would lead to such rotation curves arises naturally when the visible matter modelled as a collisionless Maxwellian gas is embedded in a dark halo of collisionless particles with a much higher dispersion in velocities
The Source Spectra and Residence Times of Galactic Cosmic Rays Derived from CALET and AMS measurements
The recent measurements of the spectral intensities of cosmic-ray nuclei have
suggested that the ratio of Boron to Carbon nuclei, , comprises two
components, one which carries all of the energy dependence and the other a
constant independent of energy per nucleon. This supports the earlier
theoretical expectations and results of gamma-ray astronomy that one of these
components is attributable to spallation in a cocoon like region surrounding
the sources and the other in the general interstellar medium before cosmic rays
leak away from the Galaxy. A new way of analyzing cosmic-ray spectra is
presented here to shed light on the recent findings: based solely on the
assumption that the B nuclei in cosmic rays are entirely the products of
spallation of heavier nuclei, we solve a cascade of propagation equations to
derive both the source-spectra of p, C, O, and Fe nuclei prior to any
spallation effects and the energy dependence of the leakage lifetime,
, implied by the energy dependence of . We find that the source
spectra of these nuclei are nearly power laws with the same index, in the
nested leaky-box model where the energy-dependent part of the matter is
traversed in a cocoon surrounding the sources, and the constant part of the
traversal is in the interstellar medium. This is not the case for the alternate
choice with the grammage in the sources being a constant. We briefly discuss
our results and comment on some aspects of cosmic-ray propagation.Comment: 13 pages, 4 figures, submitted to PR
Dynamical effects of an extended cloud of Dark Matter on dwarf spheroidals
If the dwarf spheroidals are embedded in an extended cloud of dark matter then their density profiles can be reproduced by assuming a Maxwellian distribution of velocities for the constituent stars. The observed luminosity profiles of dwarf spheroidals imply densities for the dark matter in the range 10-26 to 10-25 g cm-3, and mass-to-luminosity ratios which are typically an order of magnitude greater than those of globular clusters. Neutrinos of mass ~10 eV and <v> ~1000 km s-1 can provide this requisite density for the background
Is there an unaccounted for excess in the extragalactic cosmic radio background?
Analyses of the distribution of absolute brightness temperature over the radio sky have recently led to suggestions that there exists a substantial unexplained extragalactic radio background. Consequently, there have been numerous attempts to place constraints on plausible origins of this "excess." We suggest here that this expectation of a large extragalactic background, over and above that contributed by the sources observed in the surveys, is based on an extremely simple geometry adopted to model the Galactic emission and the procedure adopted in the estimation of the extragalactic contribution. In this paper, we derive the extragalactic radio background from wide-field radio images using a more realistic modeling of the Galactic emission and decompose the sky maps at 150, 408 and 1420 MHz into anisotropic Galactic and isotropic extragalactic components. The anisotropic Galactic component is assumed to arise from a highly flattened spheroid representing the thick disk, embedded in a spherical halo, both centered at the Galactic center, along with Galactic sources, filamentary structures and Galactic loops and spurs. All components are constrained to be positive and the optimization scheme minimizes the sky area occupied by the complex filaments. We show that in contrast with simple modeling of Galactic emission as a plane parallel slab, the more realistic modeling yields estimates for the uniform extragalactic brightness that are consistent with expectations from known extragalactic radio source populations
The implications of gamma-ray lines observed from the Orion complex
The observation of intense gamma-ray line emission from the Orion complex, attributed by Bloemen et al. to de-excitation of cosmic-ray carbon and oxygen nuclei, has important implications for emission from Orion in the infrared and in high-energy gamma-rays, and also for the theories of cosmic-ray origins. Some of these implications are briefly pointed out
Internal dynamics and dynamical friction effects in the dwarf spheroidal galaxy in fornax
In the Fornax dwarf spheroidal galaxy the globular clusters are distributed widely, without any significant central concentration. Oh et al. pointed out that such a distribution is paradoxical: dynamical friction effects estimated using single-component King models would have forced the globular clusters to spiral down to the center of the galaxy well within a Hubble time. This paper is devoted to a discussion of this paradox. We describe a model in which the stars of the dwarf spheroidal galaxy are embedded in a cloud of dark matter, and each of these components is specified by its own phase-space distribution function. This model allows us to fit self-consistently the observed luminosity profile and the spatial variation of the velocity dispersion of the stars. This fitting yields two basic parameters, related to the central density and velocity dispersion, that characterize the phase-space distribution of dark matter. The dynamical friction effects calculated on the basis of this self-consistent model are small enough that the observed spatial distribution of the globular clusters poses no difficulty, and the apparent paradox is resolved. Thus, we have at hand a model for Fornax that reproduces the main observed features of this dwarf spheroidal galaxy
Development of a Thomson X-ray Polarimeter
We describe the current status of the design and development of a Thomson
X-ray polarimeter suitable for a small satellite mission. Currently we are
considering two detector geometries, one using rectangular detectors placed on
four sides of a scattering element and the other using a single cylindrical
detector with the scattering element at the center. The rectangular detector
configuration has been fabricated and tested. The cylindrical detector is
currently under fabrication. In order to compensate any pointing offset of the
satellite, a collimator with a flat topped response has been developed that
provides a constant effective area over an angular range. We have also
developed a double crystal monochromator/polariser for the purpose of test and
calibration of the polarimeter. Preliminary test results from the developmental
activities are presented here.Comment: 4 pages, 5 figures, in "X-ray Polarimetry: A New Window in
Astrophysics", edited by R. Bellazzini, E. Costa, G. Matt and G. Tagliaferri,
Cambridge University Press (proceedings of "The Coming of Age of X-ray
Polarimetry," Rome, Italy, April 27-30, 2009